The X-mount.subdir option uses a detached-tree fast path on Linux 6.15 and later and passes the configured subdirectory to open_tree() with AT_SYMLINK_NOFOLLOW. That flag does not stop intermediate symlink traversal or keep resolution inside the newly mounted filesystem. A local unprivileged user with an fstab-authorized X-mount.subdir entry can attach a host path at the intended mountpoint.
Weakness
The product attempts to access a file based on the filename, but it does not properly prevent that filename from identifying a link or shortcut that resolves to an unintended resource.
Affected Software
| Name | Vendor | Start Version | End Version |
|---|---|---|---|
| Red Hat Hardened Images | RedHat | util-linux-main-2.42.2-3.4.hum1 | * |
| Util-linux | Ubuntu | upstream | * |
Potential Mitigations
- Follow the principle of least privilege when assigning access rights to entities in a software system.
- Denying access to a file can prevent an attacker from replacing that file with a link to a sensitive file. Ensure good compartmentalization in the system to provide protected areas that can be trusted.
Related Attack Patterns
- https://cwe.mitre.org/data/definitions/132.html
- https://cwe.mitre.org/data/definitions/17.html
- https://cwe.mitre.org/data/definitions/35.html
- https://cwe.mitre.org/data/definitions/76.html
References
- https://access.redhat.com/errata/RHSA-2026:63162
- https://access.redhat.com/security/cve/CVE-2026-78409
- https://bugzilla.redhat.com/show_bug.cgi?id=2522607
- https://github.com/util-linux/util-linux/security/advisories/GHSA-8f2p-47x3-43mv
- https://github.com/util-linux/util-linux/security/advisories/GHSA-8f2p-47x3-43mv